The term "to simulate an impulsive causal system" refers to the fact that in generating, collecting and processing seismic information using an impulsive system associated with an earth formation, such systems are based upon the assumption that the responses of the associated elements in the chain are always causal and often precisely or substantially minimum phase. The accuracy of the assumption can be attested to by the fact that processing techniques such as deconvolution yield such accurate results. But in non-impulsive generation of input vibratory waves followed by collection and processing of the received data that include steps related to the cross-correlation of the input and received signals, the elements in the aforementioned chain are not minimum phase. Such realization is seen in the fact that conventional cross-correlated records, the correlation functions are double sided in time; on one side the correlation pulses go forward in time; on the mirror image, the correlation pulses go backward as defined by a series of negative time values. In this regard, see "Reflection Seismology", Kenneth H. Waters, John Wiley & Sons, 1st Ed. at page 80 where a series of typical autocorrelation functions are described.
Likewise, the term "single-sided, with time" refers to the fact that in the non-impulsive system of the present invention, the cross-correlated final records define correlation pulses that begin at time zero exactly coinciding with the time of activation of the non-vibratory source, that is to say, without a mirror image of the correlation pulses being generated about time zero, as heretofore practiced.
The term "distortion-free, after processing" to describe the final records relates to the fact that there should be no undesired or unknown, linear or non-linear change in the waveforms either of the recorded field signals or of the final data after processing has been completed.
Prior to my invention, such distortion could be brought about (i) by the use of generating/recording techniques in the field wherein harmonics of the fundamental dissipate its energy, or (ii) accidentally, where the vibrator pad and the earth do not linearly interface.
Doty et al., U.S. Pat. No. 2,688,124, issued Aug. 31, 1954, for "Method and Apparatus for Determining Travel Time of Signals" describes the well-known VIBROSEIS.RTM. system of Continental Oil Company. In such a system, seismic waves are generated by mechanical vibrators on the earth's surface. Each of the vibrators is firmly anchored to the earth by the combined weight of the source. Peak forces in the neighborhood of 10 to 20 tons (and up to 36 tons) can be developed by the rapid, non-explosive interaction of the base-plate and piston system of each vibrator. Consequently, the weight of each vibrator is proportionally large to maintain the desired, continuous vibrator-earth contact during operations. The waves sent into the earth consist of long, sinusoidal wave trains of predetermined frequency and time duration characteristics rather than the much sharper wave impulses sent into the earth by the explosive sources used prior to the VIBROSEIS.RTM. system, or by "weight drop" methods including those provided by various impulse-coded systems, e.g., the so-called "pulse-coded" techniques.
There may be some confusion as to the differences of the signals produced by the VIBROSEIS.RTM. system and those produced by impulsive sources such as provided by exploding dynamite, exploding mixtures of propane and air, or by "weight drop" methods, including pulse-coded techniques.
It is well known that the capacity of any signal (including seismic signals) to carry information can be measured in a manner analogous to determining the volume of a container. Since volume is the product of height times width times length; similarly, information capacity of a signal is related to a product of amplitude, frequency bandwidth and the length of the signal.
Dynamite as a seismic energy source produces an input signal having considerable amplitude (height) and bandwidth, but has very short length. On the other hand, "non-impulsive" vibrations generated in the manner of a VIBROSEIS.RTM. system have limited amplitude, but such is compensated for by the long length of the input signal and a faithful, continuous reproduction of the control signal over the frequency spectrum of interest. That is to say, in the VIBROSEIS.RTM. system, the amplitude and phase spectra are carefully and continuously controlled so that the resulting energy spectra changes smoothly as a function of time. Thus, a smoothly varying output of desired frequency and duration characteristics is provided in contradistinction to the binary-coded (ON-OFF) square wave output generated by pulse-coded methods in which the energy-per-blow is substantially constant and cannot be so controlled.
A further essential part of the VIBROSEIS.RTM. system lies in the processing of the received data to produce records that tend to show short pulses representing reflections from subsurface interfaces. Such responses are provided by cross-correlating the recorded representation of the vibratory waves sent into the ground with the recorded representation of the waves received subsequently.
There is a further difference between impulsive systems wherein the injected waves are created by explosive sources, and non-impulsive systems. These differences have to do with causation. In the former, the explosion of the source establishes time zero for system operations; in the latter, the vibrations are injected into the earth over a longer time period and then facsimiles of the source output reflected at every interface in the reflection path are identified by cross-correlation. While such process can be regarded as a mechanism for seeking out and marking the individual facsimiles of the input vibrations (signals) in the received signal, the fact that a mirror image in the correlation function often results was heretofore not taken as demonstrating any deleterious effects. This was caused in part by the fact that the correlation process was known to be a convolution, with the signal turned around in time.
The use of cross-correlations, as taught by Doty et al. and many others since, has now become so well known in vibratory seismology that it will be presumed to be well known in the following parts of the present specification; and the description will concern itself only with differences from the prior art.
Erich, U.S. Pat. No. 4,234,053, for "Seismic Exploration Method Using a Rotating Eccentric Weight Seismic Source", describes an exploration method in which a rotating eccentric weight source is used (as a power impactor) to transmit a coded, non-Gaussian impulse input signal into the earth on a substantially constant energy-per-blow basis. An improved representation of the pulsed input signal is correlated with the raw seismic data to provide the field record of interest. But since the impulsive source is also only discontinuously coupled to the earth (i) the interaction of the mass of the eccentric weight source with the spring constant of the earth produces an output dominated by low frequency components and (ii) the pulse shape of the output can vary non-linearly with time. Hence, such a system is limited to those uses where a conventional VIBROSEIS.RTM. system cannot be employed.
Multi-array use of such sources is likewise limited.
Another relevant patent is that of Crook et al., U.S. Pat. No. 3,264,606, issued Aug. 2, 1966, for "Method and Apparatus for Continuous Wave Seismic Prospecting", which teaches driving of vibratory sources (in conjunction with conventional full-wave recording equipment) with pseudo-random codes which, although differing in detail from the preferred codes prescribed here, does share the desirable generic property of "a code sequence which may be represented as a reference time series having a unique auto-correlation function comprising a single major lobe having no side lobes of greater amplitude than the side lobes of the auto-correlation function of statistically unrelated noise components of the composite signal detected at said detecting location" (column 13, lines 32-44).
In my U.S. Pat. No. 4,346,461 for "Seismic Exploration Using Vibratory Sources, Sign-Bit Recording, and Processing That Maximizes the Obtained Subsurface Information", issued Aug. 24, 1982 and assigned to the assignee of the present application, I describe a non-impulsive vibratory system that uses a class of vibrator signals best characterized as Gaussian, zero-mean, and stationary, in conjunction with sign recording of both the injected and received vibrations at the sources and receivers. The stated advantages relate to the channel-capacity economy of sign-bit recording (at both the sources and receivers), and to the distortion-free quality of the final processed records.
I have now discovered that use of the above class of vibrator signals not only does not sacrifice information in the final processed records even though the data is collected by sign-bit recording methods, but such type of vibrator signals also favorably impacts seismic recording and processing operations where vibrator system distortion is present and when full-wave recording is contemplated.
Aside from the above, a paper of A. B. Cunningham, Geophysics, December 1979, Vol. 44, No. 12, pages 1901 et seq., for "Some Alternate Vibrator Signals", works out in mathematical detail expected types of cross-correlation functions from various types of vibrator sweeps, including certain types of pseudo-random sweeps, but not in the context used herein.